1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to an apparatus and a method for controlling a memory in a mobile communication system.
2. Description of the Related Art
Wireless communication systems have been developed to handle the case where a fixed wired network cannot be connected to a User Equipment (UE). Such a wireless communication system has evolved into a mobile communication system with the development of technology. The representative system of the mobile communication system is a cellular system.
The cellular system connects a node B performing communication with a UE through a wireless channel to a wired network. The representative system of such a cellular system includes a cellular mobile communication system using a Code Division Multiple Access (CDMA) scheme.
The cellular system has been developed in order to basically provide voice communication, but systems capable of providing various data services have recently emerged. Further, the amount of data requested by users has increased, requiring higher transmission speeds of large quantities of data. Accordingly, research has been conducted in order to support such requirements in a cellular system using a CDMA scheme.
In the meantime, in order to provide users with mass storage data at a high speed, research into a system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, which is different from a CDMA scheme, has been actively conducted. Many discussions have occurred to commercialize a system using an OFDM scheme. An Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group, which belongs to an international standardization body, is pursuing the establishment of an IEEE 802.16d standard in order to provide static UEs with a wireless broadband Internet service. The OFDM scheme may be defined as a two dimensional access method combining Time Division Access (TDA) technology and Frequency Division Access (FDA) technology. An IEEE Std 802.16™-2004 uses an Orthogonal Frequency Division Multiple Access (OFDMA) scheme in which an entire band is divided into subcarriers, some of which are grouped to form subchannels, and each of the subchannels is allocated to users. Thus, all UEs within a system use subchannels including subcarriers widely spread over an entire band. As a result, a system (OFDMA system) using the OFDMA scheme transmits data through some subchannels constituting a particular subchannel.
In an OFDMA system, all users connected to a node B share and use a common channel, and an interval, in which each user utilizes a channel, is allocated by a node B in each frame. Accordingly, the node B classifies access information as upward and downward access information, allocates the upward and downward access information in front of each frame, and broadcasts the allocated access information to all users.
In such an OFDMA system, access information is transmitted in each frame, which includes such information as a modulation scheme and a coding rate. FIG. 1 illustrates the conventional structure of a frame including an uplink and a downlink in an OFDMA system.
In FIG. 1, a vertical axis denotes a plurality of subchannel numbers 147 (S, S+1, S+2, . . . , S+L), and a horizontal axis (time axis) denotes the number 145 of an OFDMA symbol.
Referring to FIG. 1, the frame includes a DownLink (DL) 149 and an UpLink (UL) 153 divided through time division, and one OFDMA frame includes a plurality of OFDMA symbols (e.g. 12 OFDMA symbols). Further, one OFDMA symbol includes a plurality of subchannels (e.g. L subchannels). Such an OFDMA system denotes a communication system which aims to acquire frequency diversity gain by distributing entire subcarriers used in the system, specifically, data subcarriers, to an entire frequency band.
In the DL 149, a preamble 111 for synchronization of a transmission-side UE and a reception-side node B is disposed at the head thereof, broadcast data information including a Frame Control Header (FCH) 113, a DL_MAP 115 and an UL-MAP 117 is disposed after the preamble 111, and downlink bursts 121, 123, 125, 127 and 129 are included in symbols.
In the UL 153, preambles 131, 133 and 135 for synchronization of the transmission-side UE and the reception-side node B exist before uplink bursts 137, 139 and 141, respectively, and a ranging subchannel 143 for adjusting node B receive power exists. Information about the positions and allocation of the uplink bursts 137, 139 and 141 and the downlink bursts 121, 123, 125, 127 and 129 is transmitted from the node B to the UE through the DL_MAP 115 and the UL-MAP 117, and the UE variably receives a subchannel, in which a frequency and a symbol are combined, through the received information in each frame so as to communicate with the node B. That is, the UE uses different subchannels in each frame instead of a static subchannel.
A conversion process from the DL 149 to the UL 153 is performed during a first Transmit/receive Transition Gap (TTG) 151, and a conversion process from the UL 153 to the DL 149 is performed during a second Receive/transmit Transition Gap (RTG) 155. After the conversion processes, a preamble is provided so that the UE can acquire system synchronization.
In a conventional mobile communication system using an OFDM scheme, information about one frame is transmitted from a node B to a UE through a MAP message. The MAP message may be classified as a DL-MAP message for transferring downlink information or an UL-MAP message for transferring uplink information. In the MAP, the smallest unit reporting a piece of information will be referred to as Information Element (IE). Each IE is distinguished by a Downlink Interval Usage Code (DIUC) and an Uplink Interval Usage Code (UIUC) of four bits. Herein, DIUCs 0 to 12 and UIUCs 1 to 10 include information of a burst which is the smallest unit having transmission characteristics such as a modulation scheme and a forward error correction scheme.
FIG. 2 illustrates the construction of a receiver in a conventional mobile communication system using an OFDM scheme. Hereinafter, a memory control method in the conventional mobile communication system will be described with reference to FIG. 2.
FIG. 2 shows the simplification of a control path through which a MAP decoder 201 transfers information required by a demodulator 203 and a channel decoder 205, and a data path through which data transferred to a Low Medium Access Control (LMAC) 207 via the demodulator 203 and the channel decoder 205 is stored in a memory through information reported by a Data Receiver Block (DRB) 209, wherein the memory can be accessible by software.
A downlink burst IE includes a DIUC, an OFDMA symbol offset, a subchannel offset, boosting, the number of OFDMA symbols, the number of subchannels and a repetition coding indication field. The MAP decoder 201 extracts a burst allocated to a UE, adds burst indices in a sequence of 0, 1, 2, etc., according to bursts, and then transfers corresponding information to the demodulator 203 and the channel decoder 205. That is, since a UE can receive a plurality of bursts within one frame, the MAP decoder 201 extracts only burst-based information about a burst corresponding to the UE from the bursts, and then transfers the extracted information to blocks requiring the information. In the case of a downlink, if the channel decoder 205 performs channel decoding by using the information (i.e. location, modulation/coding, boosting and repetition) received from the MAP decoder 201, and transfers corresponding data to the LMAC 207, the LMAC 207 classifies the received data according to MAC messages. The LMAC 207 performs Header Check Sum (HCS) decoding according to MAC messages. If the situation requires, the LMAC 207 performs Cyclic Redundancy Check (CRC) check and decryption, and forms a MAC Packet Data Unit (PDU) descriptor for the results from the CRC check and decryption. Such MAC PDU descriptor and MAC messages are stored in a corresponding memory 211 based on bursts according to burst descriptors previously made by the DRB 209.
FIG. 3 illustrates one example of the burst descriptor.
FIG. 3 shows an RxBstInitAddr register 301, and a burst descriptor transferred from the DRB 209 to the LMAC 207, wherein the RxBstInitAddr register 301 is required when the LMAC 207 reads a burst descriptor and the DRB 209 writes the burst descriptor.
The burst descriptor includes a *NextBurstPtr 303, a *PduPtr 305, an F 307, a O 309, a Burst_Len 311, an L 313, a CurNum 315, an R 317 and a TotNum 319. The *NextBurstPtr 303 reports the storage position of a subsequent burst descriptor, and the *PduPtr 305 reports the storage position of a MAC PDU included in a current burst. The F 307 functions as a valid burst indicator in which 0 is written by the DRB 209, and is updated by the LMAC 207 as 1 if all MAC PDUs in a burst are transmitted. The O 309 is a current burst queue overrun flag reported as 1 while the LMAC 207 overwrites data in an existing memory when there is no longer available memory. The Burst_Len 311 indicates the entire length of MAC PDUs, which are included within a corresponding burst, by the byte, the L 313 reports that the current burst is the last burst, and the CurNum 315 indicates a current burst number. The R 317 is a reserved field currently not used, and the TotNum 319 denotes the number of entire bursts.
If the DRB 209 initially writes the first burst descriptor address in the RxBstInitAddr register 301, and connects a subsequent burst descriptor to a current chain for reporting, the LMAC 207 stores data corresponding to a burst in the memory 211 while following the RxBstInitAddr register 301 and the *NextBurstPtr 303.
Since the DRB 209 allocates a memory with a static size of each burst without separate frame information, it is impossible to efficiently use the memory. Since the burst size is not limited, and 64 DL bursts at maximum can be used in the use of a frame according to the standard, unexpected problems may occur when using the memory with a static size when there is a change in a profile including the number of bursts, the burst size and the ratio of an uplink/downlink. For example, when a size corresponding to a burst with a maximum size available for a UE is allocated to all bursts, a large memory exceeding the capacity of a system may be required, or the maximum value in the number of processible bursts may decrease.